It has been known that an exhaust gas discharged from an internal combustion engine of a vehicle such as buses, trucks or the like or a construction vehicle contains particulates having a harmful effect on environment and the human body, and techniques for removing the particulates are required. As one of such techniques, there is, for example, a honeycomb structural body (filter) for the exhaust gas purification in which the exhaust gas is passed through a porous ceramic member to catch and remove the particulates.
As a concrete embodiment of the ceramic honeycomb structural body, as shown in FIG. 10, there can be mentioned a ceramic honeycomb filter 120 obtained by combining a plurality of pillar-shaped porous ceramic members (units) 130 through sealing material layers 124 to form a ceramic block 125 and forming a sealing material layer 123 for preventing leakage of exhaust gas around the ceramic block 125.
As shown in FIG. 11(a), the honeycomb filter 120 may be made by bundling a plurality of pillar-shaped porous ceramic members 130 as a constructional unit (unit). The porous ceramic member 130 develops function as a filter by catching and removing particulates through partitions 133 when the exhaust gas passes through the partitions separating a plurality of through-holes 131 arranged side by side in the longitudinal direction.
The through-holes 131 formed in the porous ceramic member 130 have such a structure that as shown in FIG. 11(b), either one of the end portions at inlet side or outlet side of the exhaust gas is plugged with a plugging material 132 (preferably a checkered pattern) in which the exhaust gas flowed from one end portion of the opened through-hole (cell) 131a passes through the partitions 133 separating this through-holes 131a and flows into the adjacent through-hole (cell) 131b to discharge from the other end portion.
Moreover, the sealing material layer 123 arranged on the outer circumference is arranged for the purpose of preventing the leakage of the exhaust gas from the outer peripheral portion of the ceramic block 125 when the honeycomb filter 120 is placed in the exhaust passage of the internal combustion engine as mentioned above.
Such a ceramic honeycomb filter 120 is used for large-sized vehicles, diesel engine mounted vehicles and the like at present because the heat resistance is excellent and the regeneration treatment is easy. When the honeycomb filter 120 is placed in the exhaust passage of the internal combustion engine, if particulates in the exhaust gas discharged from the internal combustion engine pass through the honeycomb filter 120, they are caught by the partitions 133 (wall flow type) to attain the purification of the exhaust gas.
As this type of the exhaust gas filter, there has hitherto been proposed a filter constituted with at least two kinds of through-hole groups composed of a large volume through-hole group being relatively large in a total area of a section perpendicular to the longitudinal direction and a small volume through-hole group being relatively small in the total area of the section in which a time requiring the regeneration is prolonged by flowing the exhaust gas from the large volume through-hole group to the small volume through-hole group.
These large volume through-holes and small volume through-holes may be produced by changing an area ratio of plugging materials viewed from the cross-sectional direction when the volumes of the through-holes are equal, or by changing the volumes of the through-holes to more than two kinds and further changing the area ratio viewed from the sectional direction perpendicular to the longitudinal direction, that is, pore size of cell (area of section), or by forming two kinds of through-holes having different volumes, which are a through-hole having a large volume (large volume through-hole) and a through-hole having a small volume (small volume through-hole).
As the exhaust gas filter having such through-hole structure, there is an exhaust gas filter for a diesel engine produced by using dust metal, glass, ceramic, resin, organic polymer, paper, woven cloth, glass-ceramic mixture, thermet or the like, and particularly using cordierite as described in, for example, JP-A-3-49608.
FIG. 12 schematically shows a section of the above exhaust gas filter for diesel engine perpendicular to the longitudinal direction. This exhaust gas filter 160 has such a sectional shape that squares smaller than a regular tetragon constituting checkers are arranged at intersections of the checkers, and comprises small-volume through-holes 161b corresponding to the small squares and large-volume through-holes 161a existing therearound in which partitions 162a and 162b are formed among these through-holes.
Also, a microfilm (page 4, FIG. 6) of Japanese Utility Model Application No. 56-187890 discloses an exhaust gas filter consisting of cordierite produced by compounding silica, alumina and magnesia and rendering a cell pitch of a through-hole at the inflow side into substantially 1.0-2.5 mm.
FIG. 13 is a cross section view schematically showing a section of the above exhaust gas filter 200 perpendicular to its longitudinal direction (hereinafter referred to as “section” simply). In this exhaust gas filter 200, the sectional form is a combination of small volume through-holes 202 having a triangular shape in section around the circumference of a large volume through-hole 201 having a hexagonal shape in section.
Further, Japanese Patent No. 3130587 (page 1) discloses an exhaust gas filter made of silicon carbide in which a volume ratio of large volume through-holes is 60-70% and a volume ratio of small volume through-holes is 20-30% and a cell pitch of the large volume through-holes is approximately 2.5-5.0 mm.
Furthermore, JP-A-2001-334114 (page 6) discloses an exhaust gas filter in which a percentage of total sectional area of small volume through-holes to total sectional area of large volume through-holes is 40-120%.
FIG. 14 is a section view schematically showing a section of the above exhaust gas filter perpendicular to the longitudinal direction. In this exhaust gas filter 300, the above sectional form is a combination of small volume through-holes 302 having a section of oblong hexagon around the circumference of a large volume through-hole 301 having a section of hexagon, in which hexagonal large volume through-holes 301 and trapezoidal large volume through-holes 303 are arranged side by side in the vicinity of the outer periphery.
In the exhaust gas filters of the conventional techniques, there are existent two kinds of partitions such as a partition for directly flowing an exhaust gas in (hereinafter referred to as “direct inflow partition”) and a partition for indirectly flowing an exhaust gas in (hereinafter referred to as “indirect inflow partition”).
Therefore, when the exhaust gas filter starts to catch particulates, the exhaust gas is first caught by the direct inflow partition (FIG. 16(a)), but as the resistance caused by accumulation thickness of the particulates becomes gradually high, the exhaust gas easily flows in the indirect flow partition (FIG. 16(b)) to accumulate the particulates on the indirect inflow partition.
Locally viewing the wall portion of the filter, at an initial stage of easily catching particulates in the direct inflow partition (FIG. 16(a)-FIG. 16(b)), the flow amount of the exhaust gas is high at the direct inflow partition as compared with the indirect inflow partition. In addition, the amount of particulates caught is relatively equal or higher in the direct inflow partition, and hence violent combustion is liable to cause at the direct inflow partition.
At the next stage (FIG. 16(b)-FIG. 16(c), since the particulates are caught in the direct inflow partition, the exhaust gas hardly flows in the direct inflow partition as compared with the indirect inflow partition. That is, the flow amount of the exhaust gas becomes high in the indirect inflow partition as compared with the direct inflow partition. Noting the amount of particulates caught at this state, it is relatively much or equal in the direct inflow partition as compared with the indirect inflow partition. Therefore, the most violent combustion occurs at such a place that the flow amount of the gas is high and the amount of particulate caught is high. That is, it is considered that violent combustion easily occurs in the vicinity of a place contacting the direct inflow partition with the indirect inflow partition.
Therefore, when such a honeycomb structural body is used as a filter and regeneration treatment of such a filter is conducted, an uneven temperature distribution is caused by the combustion of particulates to locally apply a large thermal stress, so that a vital defect such as crack or the like occurs in the exhaust gas filter, and as a result, there is a problem of breaking the exhaust gas filter. Further, even if breakdown does not occur at once, thermal stress acts locally and repeatedly at the time of regeneration, so that there is a problem of lowering regeneration limit of the exhaust gas filter.
Further, when the exhaust gas filter is used over a long period of time, there is a problem of accumulating ash contained in the exhaust gas.
That is, as to the general filter, since the mechanism of particulate accumulation is constant, the ash accumulation condition is substantially constant. However, in case of changing an opening ratio (large and small volume through-holes), since the condition of particulate accumulation changes, the condition of ash accumulation is changed, so that when using over a long period of time, there are generated a wall surface accumulated with ash and a wall surface not accumulated with ash, and as a result, there is a problem that uneven ash accumulation is caused to bring about the increase of pressure loss.
The invention is made for solving the above problems inherent to the conventional techniques, and a main object thereof is to provide a honeycomb structural body having an excellent durability, which is large in the catching amount of particulates per unit volume and the no crack or the like is caused in the repetitive use over a long time of period.
Another object of the invention is to provide a honeycomb structural body, in which uneven ash accumulation hardly occurs even in the use over a long period of time.